1. Field of the Invention
This invention relates to an AC motor, such as a stepping motor which is rotated and driven by polyphase current, and to a control device therefor.
2. Related Art
Conventionally, stepping motors are known which are rotated and driven by using two phase currents of a P phase and a Q phase as disclosed in Japanese Published Unexamined Patent Application No. 2001-161055. FIG. 51 is a cross section showing a configuration of a conventional two-phase stepping motor in case of eight poles. FIG. 52 is a circumferential expansion plan of a permanent magnet provided in a rotor of the two-phase stepping motor shown in FIG. 51. FIG. 53 is a circumferential expansion plan of the stator poles of the two-phase stepping motor shown in FIG. 51.
In the conventional two-phase stepping motor shown in these figures, a positive current is passed to a P-phase winding 128. Assuming that a magnetomotive force is then generated from a permanent magnet 121 in a direction toward a stator pole 124, a rotor is rotationally transferred to a position where the stator poles 124 and the N poles of the permanent magnet 121 face with each other, and is stopped. Then, P-phase current is rendered to be zero, and a positive current is passed to a Q-phase winding 129. Assuming that a magnetomotive force is then generated from a permanent magnet 122 in a direction toward stator poles 126, the rotor is rotationally transferred to a position where the N poles of the permanent magnet 122 and the stator poles 126 face with each other, rotating at a mechanical angle of 22.5 degrees, i.e. an electrical angle of 180 degrees. Subsequently, when Q-phase current is rendered to be zero, and a negative current is passed to the P-phase winding 128, the rotor is rotationally transferred to a position where the stator poles 124 and the S poles of the permanent magnet 121 face with each other, rotating at a mechanical angle of 22.5 degrees.
Then, when the P-phase current is rendered to be zero, and a negative current is passed to the Q-phase winding 129, the rotor is rotationally transferred to a position where the stator poles 126 and the S poles of the permanent magnet 122 face with each other, rotating at a mechanical angle of 22.5 degrees. Then, when the Q-phase current is rendered to be zero and a positive current is passed to the P-phase winding 128, the rotor is rotationally transferred to a position where the stator poles 124 and the N poles of the permanent magnet 121 face with each other, rotating at a mechanical angle of 22.5. Thus, the rotor returns to the original position, ultimately rotating once. By repeating the above operations successively, the motor can be subjected to rotation control. When a reverse rotation is required, the above operations may be reversely carried out. The steps of operations described above are for the case where currents of two phases are intermittently provided. If controlling two-phase AC sinusoidal currents is effected with a phase difference of 90 degrees in electrical angle, smoother rotation control can be performed.
Motors of the type as shown in FIGS. 51, 52 and 53 are used being incorporated in various apparatuses. Thus, there has been a need for motors of higher performance, smaller size and lower costs. For this reason, magnetic circuits of the motors having conventional configurations have been improved to effectively utilize the magnetic flux generated by the permanent magnets 121 and 122. There has also been a problem that leakage of the magnetic flux between the stator poles of the P phase and those of the Q phase causes excessive torque ripples or vibration, and noises. Further, in controlling the voltage and current of a two-phase motor by a transistor inverter, unlike three-phase AC control which requires six transistors for parallel three-phase control, more number of transistors, such as eight, is required to thereby induce problematic deterioration in the driving efficiency of transistors and induce complexity in control.